The present invention relates to a configuration of an ice detector that detects ice at temperatures that are near freezing and which has a pressure field that reduces the pressure on surface regions so that such regions cool to a lower temperature as air flows past the detector to detect ice prior to formation on critical aircraft surfaces. The ice detector is used on air vehicles and provides a warning of actual ice accretion.
Existing magnetostrictive ice detectors perform well over the typical aircraft performance envelope. However, as more and more aircraft are designed with high performance wings situations may arise at temperatures near freezing where ice will form on a wing while the conventional ice detector provides no information indicating ice. The critical temperature is defined as the temperature above which no ice will form on a structure given the aircraft configuration and other atmospheric conditions. The critical temperature can be different for a typical airfoil configuration and for a conventional ice detector, at the same airspeed. The conventional ice detectors generally have a circular cross section probe.
A paper entitled “Equilibrium Temperature of an Unheated Icing Surface as a Function of Air Speed”, Messinger, B. L., J. Aeronaut. Sci., p. 29-42, Jan. 1953, provides insight into the thermodynamic balance at temperatures near the critical temperature for two dimensional cylinders. There comes a point in which the aerodynamic heating associated with direct impact cannot overcome the propensity of supercooled droplets (liquid water at temperatures below freezing) to change phase and remain on the structure as accreted ice. If the temperature is cold enough this will occur. In practice, the size of the ice detectors relative to the size of most wings can be selected so as to cause ice to accumulate on the detector faster than accretion on the wing, which is the intended result. This, however, did not take into account the fact that airflow over the lifting surface of the wing or airfoil can create localized areas of temperature colder than the ice detector. Hence ice accretion may occur on the wing at temperatures warmer than the conventional ice detector.
At high angles of attack, such as those present in takeoff and landing of an aircraft, the airflow around the leading edge of the wing accelerates around the top and creates a region of lower pressure or vacuum relative to ambient static pressure. This lower pressure in turn creates a temperature drop near the leading edge of the wing, and in the most extreme cases the area where the lower pressure occurs experiences ice accumulation. In other words, if supercooled droplets of water are present in the area of the wing where there is a lower pressure and a sufficient temperature drop occurs, ice will form.